1. Field of Invention
This invention relates to an electrochemical cell having a nonaqueous electrolyte and, particularly, to lithium cells of both primary and secondary chemistries. The activating electrolyte is provided with an organic cyclic carbonate additive.
2. Prior Art
As is well known by those skilled in the art, an implantable cardiac defibrillator is a device that requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. From time to time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected.
Lower pulse voltages from the electrochemical cell power source to the capacitor caused by voltage delay, even if only temporary, are undesirable since they can cause circuit failure in cardiac defibrillator applications. They can also effectively result in shorter cell life. Internal cell impedance (Rdc) build-up also reduces cell life by lowering the pulse voltage during high rate discharge. Accordingly, reduction and even elimination of voltage delay and Rdc during pulse discharge is important for proper cardiac defibrillation operation and extended device life.
In that light, it is well known that the anode surface film, known as solid-electrolyte interface (SEI), plays a very important role in the discharge performance of either a primary or a secondary alkali metal electrochemical cell, and in particular, a lithium cell. The formation of a surface film is unavoidable for alkali metal, and in particular, lithium metal anodes, and for lithium intercalated carbon anodes, due to their low potential and high reactivity towards organic electrolytes. The present invention is directed to modification of the chemical composition and morphology of the anode surface film to make it electrically insulating and ionically conducting.
U.S. Pat. No. 5,753,389 to Gan et al. describes several classes of organic carbonate additives that are useful in the electrolyte of lithium electrochemical cells for the purpose of modifying the SEI to improve its ionic conductivity. Among them are linear carbonates, such as dibenzyl carbonate and diallyl carbonate.

Cyclic carbonates having a five-member ring, such as 4-phenyl-1,3-dioxolan-2-one are also described in the Gan et al. patent.

The primary reason the additives of the Gan et al. patent have a beneficial effect on cell performance is that they are reductively cleaved to produce Li2CO3. This compound forms on the anode surface as a protective film, which slows down or prevents further decomposition of electrolyte at the anode surface.
In order to achieve the desired effect, the carbonate additives have to reductively compete with the electrolyte components to form Li2CO3 on the anode surface. One of the key requirements is that at least one of the C—O bonds in the carbonate [C—OC(═O)O—C] functional group is weaker than the functional groups in the electrolyte components, or that the reduction product is relatively more stable than the reduction product of the electrolyte components.
Subsequently, Crespi et al. in U.S. Pat. No. 6,017,656 described a class of cyclic carbonates that are useful in nonaqueous electrolytes for providing an ionically conductive passivation film on the anode surface. The cyclic carbonates included 4,5-diphenyl-1,3-dioxolan-2-one, among others having a five-member ring base (or being of an ethylene carbonate ring).

However, in both the above-discussed Gan et al. and Crespi et al. patents, no cyclic carbonates larger than those having a five-member ring base are described.